The present invention relates to a process for manufacturing clear shaped articles such as deli containers, lids, cups and the like from blended, nucleated polyolefin compositions.
It is desirable to produce clear or transparent shaped articles such as deli containers, lids, cups and the like so that the material inside of the container can be viewed by potential customers before purchasing. Nucleated polypropylene provides for a high degree of clarity or transparency. However, shaped articles produced from nucleated polypropylene have a tendency to warp. This is particularly true for flat, shallow-draw parts such as lids. Therefore, shaped articles produced from nucleated polypropylene can be difficult to stack, for example for use in production lines. As a result, non-nucleated materials are used for producing such shaped articles. However, non-nucleated materials do not provide for a high degree of clarity or transparency.
There are many different techniques for forming shaped articles from polyolefin compositions. The techniques are sometimes referred to as primary processes and secondary processes. Primary processes include injection molding, compression molding, and casting; while secondary processes include melt spun fiber stretching, tubular film (xe2x80x9cbubblexe2x80x9d) forming, blow molding and thermoforming.
The primary processes consist of forcing the polyolefin composition into a cavity and duplicating the cavity""s shape. In the process of casting, the cavity may be filled by gravitational flow with a low viscosity liquid (reacting monomer or prepolymer) and, following polymerization, the liquid solidifies. In compression molding, a prepolymer solid mass is heated up or melted and forced to undergo a squeezing flow by hot mold surfaces that close to form a final shape. The prepolymer usually crosslinks and assumes the shape of the closed cavity permanently. In the injection molding process a polymer melt is forced through an orifice or gate into a closed cold mold where it solidifies under pressure in the shape of the mold cavity. The polymer is melted, mixed and injected from the injection unit of the machine. Tadmor, Zehev and Gogos, Costas G., Principles of Polymer Processing, John Wiley and Sons, Inc., New York, 1979, p. 584.
With respect to secondary shaping processes, melt spun fiber stretching, tubular (xe2x80x9cbubblexe2x80x9d) forming and blow molding occur immediately after die forming. Blow molding involves using gas pressure inside an almost molten plastic tube to expand the plastic until it fills the cavity in the mold and solidifies against the walls of the mold. Thermoforming involves polymer sheets or films that have been extruded and solidified in a separate operation. Tadmor, Zehev and Gogos, Costas G., Principles of Polymer Processing, John Wiley and Sons, Inc., New York, 1979, p. 632.
Thermoforming is a generic term encompassing many techniques for producing useful shaped articles from flat sheets. Thermoforming is one of a family of processes that deal with the pressing or squeezing of pliable plastic into a final shape. Thermoforming is differentiated from the primary processes previously discussed as the primary processes involve melting of the polymer followed by solidification. For example, in injection molding, the initial resin state is pellet or powder and the shaping is done on the polymer as a liquid. Additionally, in compression molding pressures are substantially higher than those employed in traditional vacuum or low pressure thermoforming and the polymer is shaped as a liquid between matched metal molds.
Several polypropylene materials are currently used to make sheets which can be used in thermoforming. However, polypropylene is not inherently clear because of its semi-crystalline nature. While it is known to use certain additives to improve the clarity of polypropylene, certain of these known additives result in a high degree of warping.
U.S. Pat. Nos. 5,300,549 and 5,319,012 to Witco Corporation (xe2x80x9cthe Witco patentsxe2x80x9d) disclose polyolefin compositions and method and compositions for their preparation. The compositions produced have improved clarity. Further, the Witco patents describe that articles may be manufactured from the compositions by casting, compression molding or injection molding; films may be made by flowing or by extrusion; filament, bars, tapes and the like may be obtained by extrusion. The Witco patents also teach that the invention is useful in all other processes involving melting of the polymer followed by solidification.
As mentioned, thermoforming is not a process in which the polypropylene is melted and resolidified. Thermoforming is an extremely versatile process which is used to make many types of shaped articles well known to many consumers. Therefore, there is a need to produce shaped articles such as deli containers, lids, cups and the like using thermoforming processes while achieving a high degree of clarity or transparency and a low tendency to warp.
The present invention provides a process for manufacturing a shaped product from a polyolefin composition nucleated with a combination of at least one dicarboxylic acid and at least one aliphatic monocarboxylic acid, for example oleic acid, stearic acid, behenic acid, myristic acid, abietic acid, lauric acid, linoleic acid, ricinoleic acid, dihydroxystearic acid, arachidic acid, eicosenoic acid, erucic acid, tetracosenoic acid, elaidic acid and mixtures thereof, comprising the steps of primarily processing the polyolefin composition by melting and resolidification of the polymer to form a starting material for a secondary processing step, and secondarily processing the starting material into a shaped article by a process that excludes melting and resolidification.
In one embodiment of the present invention, the step of secondarily processing includes the step of thermoforming.
Further, the present invention provides a process for manufacturing a shaped product from a starting material which consists of a blended nucleated polyolefin composition comprising forming the starting material into a shaped article with a process that excludes melting and resolidification of the starting material.
The present invention discloses a nucleation system that provides good clarity in thermoformed articles, but with a lower crystallization rate than the thermoforming grades of polyolefins currently in use. The present invention enables nucleated polypropylene, which is clear, to be formed into shaped articles by processes which do not involve melting and resolidification of the polypropylene. The resulting shaped articles retain the clarity of the starting material and exhibit low warpage. Those, and other advantages and benefits, will become apparent from the Detailed Description of the Invention hereinbelow.